The present invention relates in general to satellite communication systems and in particular to a satellite communication system using one-dimensional linear ratcheting in an uplink antenna.
Satellites systems are well suited to cover large geographical areas and provide long distance wireless communication. Geostationary satellites work well to cover one specific area without any handoff overhead associated with satellite movement, however, geostationary orbits may be less convenient for low cost handheld terminals. Instead a series of low Earth orbit (LEO) satellites may be employed.
LEO satellite systems have the disadvantage that the satellite is in motion relative to the stationary or slowly moving user on or near the earth""s surface. Users are usually grouped into cells depending on the user""s geographic location. In the communications system, each cell is associated with a satellite antenna beam that transmits signals to or receives signals from the users located in a particular cell.
In prior art satellite systems, the cell-beam relationship can be described as either earth-fixed cells or satellite-fixed beams. In satellite-fixed beam systems, the beams point in fixed directions relative to the satellite body and thus sweep over the cells as the satellite moves through its orbit. As a result, the users must be reassigned to different beams frequently. There must be rapid reassignment calculations and frequent messages exchanged between the satellite and the user to coordinate the reassignment, leading to a significant overhead load on the system.
In earth-fixed cell systems, the satellite must continuously repoint the antenna beams to follow the motion of the cells as seen from the moving satellite. Implementing earth-fixed cells requires a very complex antenna that can steer many beams in two angular dimensions. Rapid reassignment calculations and overhead load are reduced at the expense of a vastly more complex antenna.
Accordingly, there is a need for an improved method and apparatus that eliminates the reassignment overhead and simplifies the antenna to a one-dimensional steering antenna.
In accordance with a first aspect of the invention, a satellite communication device is provided. The satellite communication device is preferably used in conjunction with a plurality of satellite communication devices in a non-geostationary satellite communications network. The device comprises one or more active beam forming uplink antennas employing linear cell tracking for capturing uplink signals containing data packets. Further the device comprises one or more downlink antennas capable of generating independently steerable downlink beams of data packets. Still further, the device comprises one or more intersatellite link transmitters/receivers for transmitting/receiving intersatellite data packets from a satellite in the plurality of satellite communication devices. The device also comprises a routing switch for routing data packets from an uplink antenna to a downlink antenna, from an uplink antenna to a intersatellite link transmitter, from a intersatellite link receiver to a downlink antenna, and from a intersatellite link receiver to a intersatellite link transmitter.
In a preferred embodiment, the device employs yaw or roll-yaw steering to linearize an angular track of cells through the satellite communication device footprint. Also in the preferred embodiment, the downlink antenna comprises a phased-array downlink antenna which compensates for yaw or roll-yaw satellite communication device steering. Further, the downlink antenna preferably steers downlink beams in conjunction with time division multiple access downlink formatting and provides variable rate time division multiple access service. Still further, the downlink antenna preferably steers downlink beams based on fixed cell earth addresses, wherein downlink data packets are inserted in a queue based on fixed cell earth addresses and queues are assigned to downlink beams steered to the fixed cell earth addresses in bursts. Additionally, the preferred embodiment includes an uplink antenna employing one-dimensional linear ratcheting to maintain resource allocation of uplink cells along antenna columns.
In some embodiments, the device further comprises a demodulator capable of recovering the uplink data packets from the uplink beams and a radio frequency switch matrix interconnecting the uplink beams to the demodulator. In such an instance, the radio frequency switch matrix is preferably commanded in conjunction with the linear ratcheting. Further, the routing switch may also route data packets from the demodulator to a downlink antenna, and from the demodulator to the intersatellite link transmitter.